Versatile low-alloy tool steel



United States Patent 3,295,966 VERSATILE LOW-ALLOY TOOL STEEL Gary Steven, Mount Lebanon Township, Allegheny County, Pa., assignor to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey No Drawing. Filed Apr. 30, 1964, Ser. No. 363,980 2 Claims. (Cl. 75-126) This invention relates to alloy steel, and in particular to alloy steel possessing a desirable combination of properties suiting it for use in a wide variety of tool applications, but nevertheless composed of alloying elements in relatively modest amounts, such that the total cost of the steel is considerably less than that of, for example, the high-speed tool steels or high-carbon-high-chromium steel hitherto used for some of the same or related purposes.

One object of my invention is to provide a steel that is capable of being hardened to a relatively high hardness level, such as Rockwell C 64 or 65. Prior to my invention, it has been generally believed that such hardnesses could be obtained dependably with high temperature tempering only in the relatively expensive high-speed tool steels, steels containing molybdenum and/ or tungsten in substantial quantities, e.g., having a molybdenum equivalent (content of molybdenum plus one half the content of tungsten) in the range of 4.5 to percent, usually at least 7 percent. It is important to understand that for cutting applications, even such relatively non-stringent applications as tools for cutting wood, plastics, non-ferrous metals, and annealed or softened steel, as well as for taps, tool bits, milling cutters, and reamers, it is essential that the steel used be hardenable to at least Rockwell C 63 to 65. Such hardnesses have hitherto been obtained only by using high-speed tool steels, costing about a dollar per pound or more. Certain hot-work steels are known that cost about half as much as the above-mentioned highspeed tool steels and can be heat-treated by secondaryhardening to a maximum achievable hardness of about Rockwell C 59, but it is generally accepted in the tool steel industry that such steels are not suitable for the applications mentioned above. Accordingly, it is one object of my invention to provide a steel suitable for the above-mentioned cutting applications, yet about one-half the cost of the high-speed tool steels.

In the tool-steel industry, it is also known that there are other applications, such as heavy-duty punches, arbors for carbide tools, thread-rolling dies, rolls for Sendzimir mills, and components of bearings, that require a steel hardened to a hardness of Rockwell C 62 to 64. Steels for these applications must exhibit high compressive strength, such as an 0.2 percent offset compressive yield strength of about 400,000 p.s.i. or more at Rockwell C 64. As will be explained more fully below, attempts have been made to meet these applications by using steels of relatively low molybdenum-equivalent content, such as up to about 1.5 percent, classified in the AISI listing as Air Hardening Steels, but the desirable high compressive yield strengths mentioned above are not obtained with these steels. Such steels must be used in the low-tempered condition to retain the hardness of Rockwell C 62 to 64, and depending upon the direction of the residual stresses remaining in the steel after heat treatment, compressive yield strengths on the order of 240,000 to 340,000 p.s.i. are obtained. It has hitherto been necessary to accept the lower performance of the air-hardening grades or if 400,000 p.s.i. was essential to the proper performance of the tool, to use one of the relatively costly highspeed steels. It is accordingly another object of my invention to provide a steel suitable for the most stringent of the above-mentioned applications but considerably less expensive than the high-speed steels hitherto required for optimum performance in such applications.

3,295,966 Patented Jan. 3, 1967 Yet another application of tool steels is for knives, such as in high-temperature shears, slitters, and cold-metal shears. For this purpose, it is customary to use tool steels hardened to Rockwell C 58 to 60. Accordingly, it is another object of my invention to provide a higher performance steel suitable for such uses, yet sufiiciently inexpensive to compete with the lower molybdenum-equivalent steels hitherto used for such purposes.

It is yet another object of my invention to provide a steel useful as a die for the forging of difiicult-to-forge materials. The versatile new steel of the present invention competes in this respect with a large number of hotwork die steels and high-temperature alloys.

Another object of my invention is to provide a steel exhibiting the properties indicated above, along with the capability of being sulfurized to improve free-machining properties.

Still another object of my invention is to provide a steel having, in combination with the above-mentioned properties, the ability to be rolled to the form of sheet or strip, which may be subsequently treated to high strength for such applications as hack saw blades and band-saw strip.

These and other objects of the invention will become apparent from the following description and claims.

In brief summary, the invention consists in adding small amounts of tungsten, such as about 0.20 percent, to semi-high-speed steel, particularly such steel containing under 1.5 percent vanadium and over about 1.5 percent silicon, to avoid grain coarsening of the steel during austenitization and the consequent embrittlement that has hitherto prevented semi-high-speed steel from finding extensive use. In accordance with this invention, I provide a semi-high-speed steel with a critical and substantial content of silicon, about 2 percent, sufficient to improve the as-tempered hardness but not enough to cause grain coarsening. In its most limited aspects, the invention comprises steels of the character above-indicated with small additions of titanium and/or sulfur, for purposes hereinafter stated.

I have discovered that the above-indicated invention can be practiced and the foregoing objects can be achieved by providing a steel having a chemical composition as indicated in the following table, wherein, as in the remainder of the specification and claims, the numbers refer to weight percent.

ties in minor amounts not detrimentally affecting the properties.

Reasons for composition With carbon contents lower than about 0.90 percent, the attainable secondary hardness is too low. With more than about 1.25 or 1.30 percent of carbon, the steel contains excessive amounts of retained austenite and is consequently not harden'able to the high strength levels desired. In general, carbon contents lower than 1.25 percent are preferred because carbon lowers the solidus temperature (the temperature of incipient fusion) and hence contributes further to the likelihood of the development of a coarse grain structure during austenitizmg.

It has been generally believed that steels containing substantial amounts of silicon exhibit two salient drawbacks; poorer cleanliness and greater tendency to lose surface carbon during heat treatment. I have discovered, however, that the steels of the present invention exhibit neither of the above-mentioned drawbacks. I have also discovered that with silicon contents of less than about 1.50 percent, the attainable secondary hardness is low by about 1.5 points Rockwell C, but with silicon contents greater than about 2.5 percent, the steels tend to develop a coarse grain structure when austenitized at temperatures of about 2000 F. and are consequently brittle.

When the content of chromium is too low, the steel lacks adequate hardenability, whereas if the content of chromium is too high, the transformation from austenite to martensite during heat treatment tends to become sluggish; moreover, with high chromium contents, the cost of the steel is increased without substantial added benefit in properties.

In line with the objective of providing a versatile steel, it is desirable for certain purposes that the steel exhibit substantial wear resistance, and accordingly it is highly desirable that the steel contain at least about 0.75 percent, preferably 1.0 percent, of vanadium, since lower wear resistance is observed if the vanadium content is less. The steel becomes rather difficult to grind if the vanadium content exceeds the upper limits specified above, but the board idea of adding tungsten in small amounts for grain refinement is applicable with steels containing up to about 2 percent of vanadium. Steels containing more than 2 percent vanadium usually have sufi'lcient alloy carbides that the formation of coarse grains upon austenitization is not a serious problem.

Substantial difficulties of various kinds (retained austenite, brittleness, etc.) are observed if the manganese content is permitted to exceed the upper limit specified above.

It has been observed, somewhat surprisingly, that if the content in tungsten is permitted to diminish below about 0.10 or 0.15 percent, the steel tends to develop a coarse grain size upon austenitization, which leads to brittleness in the steel as heat treated. When the steel contains less than 0.015 percent of tungsten, a coarse grain size is observed, even with austenitizing temperatures as low as 2000" F. Amounts of tungsten greater than about 0.5 percent are not required, the added benefits of such higher tungsten contents not justifying the increased cost necessitated by their inclusion.

Although for most purposes a steel containing not over about 0.025 percent of sulfur is satisfactory, it may be desirable in certain instances to add up to 0.35 percent of sulfur, to impart free-machining properties. In general, if a free-machining steel is desired, adequate improvement in the machining properties can be obtained with an addition of sulfur of slightly less than 0.15 percent, so that for most purposes greater additions are not required, and additions of over about 0.35 percent are to be avoided in any event because they make the steel brittle and diflicult to hot-work.

With low molybdenum contents, the secondary hardness (hardness after quenching and tempering) is low. In its broadest aspect, my invention is applicable with steels containing as much as 5 percent of molybdenum, but with molybdenum contents greater than about 3 percent, the benefits obtained do not usually justify the added cost.

Although additions of titanium are not absolutely necessary, it is preferred to add about 0.20 percent of titanium or slightly more, to add to the wear resistance and reinforce the resistance to the formation of coarse grain structure upon austenitization. It is also preferred, however, not to add more than the upper limit of titanium specified above, because at that point the addition of the titanium becomes difficult to control, and the addition of further quantities is without substantial benefit to the properties justifying theaddition. For bearing applications, a vacuum-melted titanium-free steel is preferred.

It is within the scope of the invention to add small amounts of other elements, among which may be mentioned phosphorus, copper, nickel, columbium, tantalum, boron, hafnium, zirconium, nitrogen, uranium, tellurium,

bismuth, and the rare earth elements, for special purposes.

Usual steelmaking impurities or tramp elements, such as sulfur, phosphorous, nickel, copper, tin, arsenic, hydrogen, etc. may be present in usual amounts not inconsistent with good steelmaking practice, and otherwise the balance of the steel is iron.

Heat treatment It will be understood that the heat treatment to be used with the steel of this invention will depend upon a variety of factors, including (1) the intended use of the steel, (2) the section size, (3) the hardness to be de veloped and (4) the exact chemical composition of the steel. In general, however, the steel is heat-treatable to maximum hardness by using an austenitizing temperature in the range about 1950" F. to '2050 F., preferably about 2000 F., followed by a double or triple temper, about 2+2 hours or 2+2+2 hours, at a temperature of about 950 F. to 1025 F., preferably about 975 F. or 1000 F. When higher austenitizing temperatures, such as 2075 F. or higher, are used, the steels tend to develop -a duplex grain size (that is, some coarse grains tend to develop, causing brittleness in the product). With lower austenitizing temperature, however, not enough of the carbides in the steel are dissolved, and consequently the steel exhibits less secondary-hardening response. In view of the importance, in some applications, of obtaining maximum hardness it is essential that substantially equilibrium dissolution of carbides in the matrix during austenitization be achieved, since otherwise the secondary-hardening response will be inadequate to achieve the high strength levels desired. To achieve lower desired hardness levels, it will be desirable to use lower austenitizing temperatures, as low as 1800 F., and to temper at somewhat higher temperatures, such as 1050 to 1150 F., as Will be more particularly indicated below in the section headed Results.

It has also been discovered that in providing steels for a particular application, namely shanks for tungstencarbide-tipped tools, it is desirable to preheat the tool for about one minute at about 1020 F. before applying the brazing heat of about 1300 F., to prevent excessive softening of the shank steel.

Results The response to tempering of a steel in accordance with my invention is indicated in Table I below. The steel had the following chemical composition: 1.12 percent carbon,-0.3 percent manganese, 1.95 percent silicon, 3.87 percent chrominum, 1.09 percent vanadium, 0.30 percent tungsten, 2.59 percent molybdenum, 0.15 percent titanium and the balance iron except for incidental impurities. Laboratory-size speciments (about inch square by /2 inch long) were prepared from a 2,000-pound air induction-furnace heat of the steel. The specimens were austenitized for four minutes at the indicated temperature, air-cooled to room temperature, tested for hardness and grain size, tempered as indicated, and again tested for hardness.

1 Duplex grain size observed.

1 Triple-tempered at 1,025 F., 2+2+2 hr.

With small section sizes, such as up to about 4 inches, the as-tempered hardnesses at a given austenitizing temperature are essentially the same, whether the steel is aircooled (as is usual for such applications as rolls or threadrolling dies) or oil-quenched (as is usual for cutting tools).

Use of austenitizing temperatures higher than about 2050 F. is generally undesirable. The above results indicate that with such austenitizing temperatures the Snyder-Graif grain size may be coarser than is tolerated in accordance with commercial standards and the impact resistance drops appreciably. Moreover, the steel tends to contain retained austenite after tempering unless one uses a relatively severe tempering practice, such as triple tempering two hours each time at 1050 F., yet hardnesses the same or higher can be obtained, for example, by double tempering two hours each time at 1000 P. if a lower austenitizing temperature is used, and the steel is completely martensitic as so tempered.

Another requirement of steels for cutting-tool applications is adequate notch toughness (impact resistance). My new steels possess this requirement, as indicated from the results presented below in Table II. Charpy C-notch specimens were rough-machined from annealed bar stock having the chemical composition mentioned above, austenitized and tempered as indicated below and tested, at room temperature and at 1000 F. The C-notch specimens were prepared as described in the article C-Notch Impact Test for Steel at High Hardnesses by G. Steven and J. R. Handyside, Proceedings of the American Society for Testing Materials, 1963. e As will be apparent, higher impact-strength values are obtained with lower hardness, and vice versa, and only the value for the specimens austenitized at 2000 F. and 2050 F. and triple-tempered are typical for steels of my invention heat treated to high hardness for use in cutting applications, but the value obtained, 10.0 to 13.0 ft.lb., is adequate for a steel for such purposes.

TABLE II Charpy C-Notch Impact Strength, Tempering ft.-lb. Aust. Temp. and Temp, F. Hardness,

Time 1 2+2 Hr. Re Room 1,000 F.

Temp.

1,700 F., 1 hr 950 60 14 1, 150 49 27 1, 200 42 to 44. 37 1,800 F., 1 h! 1, 000 60 to 61.. 17. 5 1,150 50 to 51... 31.0 1,200 44 to 45--. 40.0

1,900 F., 1 hr 1e 28 41 2000 F. 4 min 10 2,000 F. 4 min 13 2,050 F. (O), 4min 11 1 All specimens were air-cooled except those marked (0) which were oil-quenched.

2 Triple-tempered at the indicated temperature, 2 hr. each time.

The steel of this invention performs satisfactorily in many cutting applications not so stringent as to require the use of high-speed steel. For example, in a tool-life test, machining AISI Type H-13 steel at Rockwell C 30 at 65 surface feet per minute in a single-point lathe out under a heavy flow of soluble cutting oil, with a cut depth of 0.062 inch and a feed of 0.010 inch per revolution, a tool of the steep composition given above, heat-treated by austenitizing for four minutes at 2050 F. and then tempering to Rockwell C 65.0 exhibited a life of 49 minutes. Machining AISI Type 4340 steel at Rockwell C 20 at 70 surface feet per minute, other conditions the same, the tool exhibited a life of 37.3 minutes. In other tests on AISI Type 4340 steel, the conditions being the same except for the heat treatment of the tool, similar results were obtained; with an austenitizing temperature of 2000 F. and a tempered hardness of Rockwell C 63.7, lives of 31.2 minutes and 27.4 minutes were observed, and with an austenitizing temperature and time of 1950" F. for 30 minutes and a tempered hardness of Rockwell C 64.0, lives of 38.0 minutes and 29.4 minutes were observed. These results are impressive in view of comparative results with two commercial high-speed steels, hardened conventionally and used to machine AISI Type 4340 steel at Rock well C 20 and 70 surface feet per minute; these commericial high-speed steels are about twice as expensive as the steel of my invention, but they exhibited average tool lives of 35 minutes and 40 minutes.

The foregoing results show that my new steel is capable, when properly heat-treated, of developing the high hardenss required (about Rockwell C 63 to 65) for use in cutting-tool applications requiring less than the very high hardnesses (R 66 and over) achievable only with the expensive high-speed steels. My new steel also meets the requirements of fine grain size, freedom from retained austenite, and adequate impact strength, and as shown above, it affords in certain applications a tool life sufficient to reduce the cost of certain cutting operations where conventional high-speed steels have hitherto been used.

My new steel is also useful for applications requiring a steel exhibiting a high compressive yield strength, such as about 400,000 p.s.i. or more at 0.2 percent offset, when heat-treated to a hardness in the range Rockwell C 62 to 64. Specimens 0.500 inch in diameter by 1.500 inch long were prepared, heat-treated as indicated below, and tested, the results being presented in Table III.

TAB LE III Compressive Yield Strength at 0.2% Offset,

p.s.i.

2,000 F., 4 min., air'cool, 1,000 F.,

2050 F., 4 min., air-cool, 1,000 F.,

The observed yield-strength values are good, greatly exceeding values of 240,000 to 340,000 p.s.i. obtained with the air-hardening AISI Type A2 and D2 steels, respectively, heat-treated to about Rockwell C 63 in the manner conventional for such steels, and approaching the values of 420,000 and 413,000 p.s.i. for conventionally hardened AISI Types M1 and M2 steels, respectively.

From Table 11 above, it will be apparent that my new steel can be heat-treated to a hardness level of about Rockwell C 58 to 60, for use as a knife steel.

As a die material for forging high-strength metals, my new steel must exhibit a combination of wear resistance, impact strength, and temper resistance. The data on impact strength and temper resistance are presented in Table II. Comparative wear-resistance tests have been conducted, the results of which are presented below in Table IV. The procedure used in these tests comprised dead-weig ht-loading a water-cooled cylindrical test speci- V=(C) (R), where V=Wear volume (in. 10" R=number of revolutions, and C=wear coefiicient for the steel tested.

comparative tests numbers indicate Values of the coefiicients observed in appear in Table IV, wherein lower better wear resistance.

TABLE IV Steel Hardness CoetIieient This invention 63 5 AISI Type M-ZH AISI Type D2 AISI Type M-l- AISI Type A-2- :ooooueo ocozroo Tests were conducted to investigate the machinability of the steel of this invention, both plain and sulfurized to improve machinability, in comparison with that of other high-speed and air-hardening steels. The test specimens were annealed bars of the steel inch thick, and the time required to penetrate the steel At-inch using a 4-inch drill, ISO-pound thrust, 460 r.p.m., was observed in each case. AISI Type M steel was taken as a standard, and in each case the observed time was divided into the time required for penetration of Type M2 steel and then multiplied by 100 to afford a Machinability Rating.

The results appear below in Table V.

The foregoing results demonstrate the wide usefulness and versatility of the new semi-high-speed steels of my invention. It will be observed, however, that the data :above are limited to results with steels of two stated chemical compositions, set forth in the following Table VI as Steels 63-293 and 63-294, together with other steels that 1 have tested and other steels known in the prior art. In the Table, the steels designated Class 360 through Class 368, inclusive, are semi-high speed steels disclosed in Tool Steels, by G. A. Roberts et al., page 415 et seq. (Third Edition, published by American Society for Metals, Metals Park, Ohio, 1962). As pointed out by Roberts et al., the semi-high-speed steels known prior to this invention have the drawback that to avoid undesirable .grain coarsening during austenitizing, they must be austenitize-d at temperatures below those which yield the highest as-tempered hardnesses. Roberts et 'al. attribute the rapid grain coarsening of such steels to their having a lower alloy carbide content than the true high-speed steels, and they suggest no way of overcoming this disadvantage, which has prevented known semi-high-speed 8 steels from having broad usefulness. Inspection of the following Table VI reveals that Steels 63-293 and 63-294 differ from the steels designated Class 360 through Class 368, inclusive, in two major respects: (1) higher silicon content and (2) necessary addition of a small amount of tungsten. A principal unexpected novel teaching, in accordance with this invention, is that the grain-coarsening problem in such steels can be overcome with a small addition of tungsten such as about 0.1 to 0.5 percent, or slightly more if desired. A further unexpected teaching is that contrary to the general belief that higher silicon contents in such steels would be detrimental as respects cleanliness and tendency to decarburize and without compensating benefits, I have discovered that a tungstentreated semi-hi-gh-speed steel can be improved in respect to hardness attainable after tempering by the addition of a substantial amount of silicon, such as 1.5 to 2.5 percent.

This silicon addition, together with the solution to the.

grain-size problem afforded by the small tungsten addition, yields a steel that possesses the combined properties required in a steel to replace the true high-speed steels in a wide variety of uses.

TAB LE VI Steel 0 Mn S'i Cr V W Mo Ti S 63293 1. 12 0.27 1. 95 3. 87 1. 07 63-294. 1. 12 0. 30 l. 98 3. 95 1. 09 Class 360 0. 0. 25 0. 25 4.00 1. 10 Class 361. 0. 90 0. 25 0. 25 4. 00 1. 90 Class 362. 1. 20 0. 25 0. 25 4. 00 3. 15 Class 353. 1. 40 0. 25 0. 25 4. 00 4. 15 Class 364 0. 0. 25 0. 25 4. 00 2. 30 2. 80 Class 365 0. 90 0. 25 0. 25 4. 00 2. 25 1. 00 Class 366 1. 20 0. 25 0. 25 4. 00 2. 90 1. 40 Class 367 0. 95 0. 25 0. 25 4. 00 2. 20 1. 90 1. 10 0. 25 0. 25 4. 00 4. 00 2. 50 2. 1. 22 0. 45 1. 94 5. O4 1. 15 0. 003 1. l. 01 0. 28 0. 26 3. 41 1. 15 0.003 2. 0. 99 0. 30 2. 10 3. 43 1. 15 0. 02 2.61 1. 09 0. 33 2. 07 2. 77 1. 12 0. 24 2. 58 0. 24 1. 12 0. 34 2. 05 3. 74 1. 25 0. 009 2. 47 0. 23 1. 08 0. 32 3. 03 4. 07 I. 15 0. 012 2. 50 0. 23

In the foregoing Table, Steels 63-293, 63-294, and 64-7 are considered to fall within the scope of the invention in its broadest aspect, and the remaining alloys are set forth for purposes of comparison.

The effect of the tungsten addition upon the grain size is shown in Table VII below.

TABLE VII Snyder-Graft Grain Size with Austenitlzing Temperature F. oi Tungsten, Steel percent 1 Duplex grain size observed.

The foregoing data reveal that without a certain critical, effective amount of tungsten, about 0.015 percent, or more, the grain size upon austenitizing at 2050 F. becomes undesirably coarse (lower than 12). Higher austenitizing temperatures give coarser grain size, and in general the steels of this invention are limited to austenitizing temperatures of 205 0 F. or less. As the results with Steel 63-293 indicate, however, it may sometimes be possible to use austenitizing temperatures of 2100 F. or possibly higher, especially if a substantial small addition of tungsten, such as about 0.30 percent, is used. Additions of greater than about 0.50 percent are contraindicated on account of cost.

Steel 62-147 exhibited fine grain size despite its low tungsten content, but this is believed to be attributable at least in part to the low silicon content of the steel. In any event, additions of small amounts of tungsten such as 9 0.015 percent or more serve to insure against the development of coarse grains during austenitizing, and as will be shown below, an addition of silicon (in an amount that would make such a tungsten addition necessary) is re- 10 (11) The ability to be heat-treated to a working hardness of Rockwell C 60 or more by austenitization at 1950 F. to 2100 F. and subsequent double or triple tempering, about 2+2 hours or 2+2+2 hours, at a temquired for steel of optimal versatility. perauire of about to 1100.

Steel 63 366 180 out med O t hjghl v1 0 While I have disclosed certain embodlments of my a c a 511C a e e invention, I intend to cover as well any change or modipercent), and the results show that it exhibited a coarse fication therein which may be made Without departing grain size with an austenitizing temperature as low as from the Spirit and Scope of the invention 1950 F. Hence, it is essential that the steels of the ind i vention not contain silicon in excess of about 2.5 percent. 1 Semi high-speed t l bl f being austenitized The effect of the silicon addition upon attainable hardat about 2050 F. Without the development of a grainness after tempering is shown in Table VIII below. size coarser than Snyder-Graft No. 12, said steel being TABLE VIII Hardness, Re, After Heat Treatment Steel 0 M11 Si Or V W Mo Ti 1. 01 0. 28 0. 26 3. 41 1. 15 0. 003 2. 73 61. 5 62. 0 63. 0 1. 16 0. 31 0.88 3. 93 1. 12 2. 54 0. 2o 62. 5 63. 5 1. 12 0. 27 1. 95 3.87 1. 07 0. 30 2. 59 0. 15 62. 5 64. 0 64. 5 1.11 0. 1. 9s 4. 74 1. 2s 0. 011 2.50 0. 23 63. 0 64. 5 0. 99 0. 30 2. 10 3. 43 1. 15 0. 02 2. 61 63. 0 64. 0 64. 5 1. 08 0. 32 3. 03 4. 07 1. 15 0. 012 2.50 0. 23 61. 5 63. 5

Heat Treatment 1Austenitized 1,950 F., Temper 1,000 F., 2+2+2 hr. Heat Treatment 2Austenitized 2,000" F., Temper 1,000 F., 2+2+2 hr. Heat Treatment 3-Austenitized 2,050 F., Temper 1,000 F., 2+2+2 hr.

The above results show that with a silicon content less than about 1.75 percent, the attainable hardness is less, by about 1.5 or 2 points Rockwell C, than with such a silicon content. This diiference appears small, but in a cutting application requiring a hardness of Rockwell C 64 or 65, the addition of at least about 1.50 percent silicon is necessary, and it is accordingly a critical feature of the preferred embodiment of my invention that the steel contain about 1.75 to 2.5 percent of silicon. The upper silicon limit is critical because of the grain-coarsening tendency of silicon, mentioned above.

Miscellaneous In the claims, the term semi-high-speed steel will be understood as meaning a steel having all the following characteristics:

(1) A carbon content of 0.75 to 1.30 percent;

(6) A silicon content of up to 2.5 percent;

(7) A chromium content of 2.5 to 5.0 percent; (8) A vanadium content of 0.75 to 2 percent; (9) A titanium content of up to 0.4 percent; (10) Balance substantially iron.

characterized by improved attainable hardness and consistiug essentially of about:

Tungsten More than 0.15% and up to 0.50%.

Carbon 0.85 to 1.3%.

Silicon About 1.5 to 2.5%.

Chromium 2.5 to 5.0%.

Vanadium 0.75 to 1.5%.

Molybdenum 2.0 to 5.0%.

Sulfur Up to 0.35%.

Titanium Up to 0.4%.

Iron Balance.

2. Steel consisting of about: 1.12% carbon, 0.27% manganese, 1.95% silicon, 3.87% chromium, 1.07% vanadium,' 0.30% tungsten, 2.59% molybdenum, balance iron.

References Cited by the Examiner UNITED STATES PATENTS 1,496,980 6/ 1924 Armstrong 126 2,753,260 7/1956 Payson 75-126 OTHER REFERENCES Tool Steels, 3rd edition, published by American Society for Metals 1962, pp. 461-465 and 718.

DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. SEMI-HIGH-SPEED STEEL CAPABLE OF BEING AUSTENITIZED AT ABOUT 2050*F. WITHOUT THE DEVELOPMENT OF A GRAINSIZE COARSER THAN SNYDER-GRAFF NO. 12, SAID STEEL BEING CHARACTERIZED BY IMPROVED ATTAINABLE HARDNESS AND CONSISTING ESSENTIALLY OF ABOUT: 